The invention concerns a method for compensation of temperature in a pressure sensor of the kind designed to convert a detected pressure into an electric signal which is registered by a measuring device.
A prior-art method used for this purpose consists of applying a number of resistances on a pressure-sensitive diaphragm, which resistances change their resistance values in response to changes in the diaphragm configuration when the diaphragm is exposed to pressure changes. As a rule, the resistances are electrically connected in a bridge circuit, of the kind known as a Wheatstone bridge.
All transducers, independently of whether they comprise resistances manufactured from thin films or thick films on an insulating material or through ion implantation in silica materials (known as piezoresistive transducers), suffer from the disadvantage of being extremely temperature-dependent. This means that the transducer output signal which is representative of a pressure at a certain moment, changes as the temperature changes, also when the pressure remains constant. This temperature dependency could, in some cases, be extremely large.
When a pressure transducer is used in applications where the temperature variations are comparatively small, it may be calibrated once the environmental conditions have stabilized somewhat. Thereafter, the effects of temperature changes are essentially negligible.
Other prior-art methods of temperature compensation consist of connecting resistances having known temperature charactersistics in different diagonals of the bridge, or of actively compensating the output signal for temperature influence by means of electronics built into the transducer. However, in this case also, the electronics are exposed to the temperature variations and consequently these variations need again be considered. As long as the temperature variations remain small it is, however, possible to obtain acceptable measurement results by any one of the above methods.
In thermal sterilization by means of vapor in an autoclave the conditions are, on the other hand, very difficult to master because during one sequence the pressure varies from between approximately 30 mbar absolute pressure and approximately 5 bar absolute pressure, i.e. a pressure ratio of approximately 1:150. At the same time, the temperature varies between approximately 20.degree. C. and 140.degree. C. In addition, the changeovers between maximum and minimum values and vice versa, both respect to pressure and temperature, take place rapidly, and several times, over a comparitively short period, during one process.
However, the above-mentioned prior-art stabilizing methods are quite insufficient in this application to meet the requirements as to accuracy with respect to the precision of the measured values in sterilization. Particularly in the case of low pressures the percentage deviations of measured values are quite considerable, often several hundred percent. At the same time, low pressures are an important measuring range in this connection.
For such an extreme activity as vapor sterilization various measures therefore have been taken to prevent the effects of temperature on the pressure sensor. Examples of such measures are isolation of the pressure sensor from the autoclave enclosure where the pressure is measured, transfer of the pressure to the sensor by way of, for instance, an oil buffer, a water bag or the use of capillary tubes. Another way is to cool the pressure sensor to prevent the pressure sensor from following the temperature increase inside the autoclave enclosure.
However, all these various previously known methods of preventing a temperature rise in the pressure sensor cause cavities to form between the autoclave chamber and the sensor. Such cavities are not desired, and normally they are not accepted by established standard specifications, because inside them an environment is created that favors collection and growth of micro-organisms. Instead, a desired end is to be able to position the sensing diaphragm of the pressure sensor directly in the wall of the autoclave enclosure, without using any intermediary tubings or pipings.
Of course, it is always possible to position a separate temperature sensor on or adjacent the diaphragm for the purpose of measuring the existing temperature during the pressure sensing operation. However, this does not solve the problem, since the pressure sensor only provides information on the temperature at its place of mounting on the diaphragm and not on the actual effects to which the pressure-sensitive resistances are exposed.